With the increasing need for bandwidth, Time Division Multiplex (TDM) networks face limits in terms of scalability, operational cost and maintenance. This results in telecommunications carriers having to replace circuit-based transport with packet-based (Ethernet) transport to realize cost and operational efficiencies, meet increasing bandwidth demands from customers at reasonable price points, bring a new level of flexibility and dynamic configuration to the network, and offer more tiered and time-based or on-demand services.
Synchronization is critical in the transition from circuit-based TDM networks to packet-based networks and, in turn, the deployment of Carrier Ethernet technology. One of the technical challenges holding back the deployment of Carrier Ethernet has been the requirement for very accurate clock synchronization in the network between source and destination, which is an absolutely essential capability for delivering wireless backhaul and leased line services. Traditionally, these services have been delivered over synchronous technologies like T1/E1 and SONET/SDH. Ethernet networks, however, are asynchronous, designed originally to deliver data and without the need for accurate clock synchronization and distribution capabilities in the network.
Without proper frequency synchronization, packet networks carrying timing-sensitive services can generate excessive jitter and wander when interfacing to TDM devices. Network-wide frequency synchronization is a new requirement driven by performance measurement, service assurance and real-time services in next generation networks. Service providers need to meet timing (frequency synchronization) requirements for Circuit Emulation Services (CES) and other services over packet switched networks that isolate remote network elements from their source of synchronization. Mobile operators need to ensure they can support the synchronization accuracy needed to avoid dropped calls and maintain quality of service (QoS).
Time (i.e., time of day or wall-clock) synchronization is inherently important to the function of communication networks. It provides a common time reference for all devices (switches, routers, gateways, etc.) on the network. Without synchronized time, accurately correlating information between devices becomes difficult, if not impossible. In the area of network security, if a network engineer cannot successfully compare logs between each of the routers and all the network servers, it is very hard to develop a reliable picture of an incident.
Presently, timing in telecom networks is delivered over synchronous technologies like T1/E1, SONET/SDH, and Global Positioning Systems (GPS). As a result, carriers rely on expensive solutions such as circuit-based T1/E1 connections and GPS receivers to ensure accurate synchronization of services across packet networks. All of these existing timing methods involve considerable capital investment for hardware at a large number of customer sites or base stations. For example, a GPS receiver is installed at each base station and used as a stable clock reference for re-timing the CES packets between the CES interface and the base station T1/E1 input. The timing signal received by the base station is retimed to be precise and stable. However, the disadvantage of GPS-based re-timers is that they involve a substantial cost and implementation burden. There is the need to equip each base station with a GPS receiver, involving a significant capital cost. With several million base stations in the world, the required investment is substantial. Another concern is that the existing GPS may not be an acceptable solution for all sites since GPS signals may be weak indoors or in metropolitan areas. Moreover, some wireless operators intentionally may not want to use a GPS signal controlled by the United States.
For these reasons, telecommunications providers have been seeking alternative solutions that would eliminate these expenses. With recent technological developments, a growing possibility has come to be delivering time and frequency synchronization over the packet-based network. Such alternative synchronization solutions over packet technology enable time and frequency synchronization to be distributed across asynchronous Ethernet, IP, and MPLS packet networks. Carriers can lower their operating costs by eliminating GPS receivers and T1/E1 connections, while maintaining high-quality service for time-sensitive applications.
Many service providers have looked at Network Time Protocol (NTP), the most popular protocol for time synchronization over LANs and WANs. NTP, however, currently does not meet the accuracy requirements for telecom grade time and frequency synchronization. The problem is that NTP packets go through the Ethernet physical and Media Access Control (MAC) layers in the switches or routers like any other packets, so synchronization is not addressed until the packets reach the end-system software stack. The synchronization signals are thus delayed by a non-deterministic amount depending on the operating system latency.
Another protocol that promises high accuracy timing delivery is the IEEE 1588 Precision Time Protocol (PTP). The main obstacle to the adoption of IEEE 1588 is that the protocol cannot be seamlessly implemented in current/existing native Ethernet interface cards. Networks requiring this protocol would have to replace these cards with IEEE 1588 compliant cards. The result is the implementation of a protocol that has a cost associated with it that the network engineer may not be willing to incur.
Exact time synchronization over packet networks is difficult to achieve, particularly, when there are uncertainties on transmission delays in the network, and on processing delays in the protocol layers of the end-systems. Packet Delay Variation (PDV) is a main cause of poor clock synchronization in communication networks. PDV must be properly mitigated when transporting timing-sensitive traffic over packet networks.
What is therefore needed is a method and system that allows end-systems attempting to synchronize with a reference source to minimize the timing uncertainties, including PDV, in order to allow the end-systems to remain time and frequency synchronized with the reference source.